In spread-spectrum systems like the WCDMA system, the spectrum is spread by introducing additional modulation with spectrum spreading (SS) codes. An SS code is a sequence consisting of so-called chips. Orthogonal code sequences are used for the SS codes, whose characteristics vary depending on the purpose of the system. With multiplication of a PN (SS) code with the data signal, the spectrum spreads according to the spectrum bandwidth of the SS code. The multiplication with the PN code in the time domain results in a convolution integral in the frequency domain. If an SS code with a length of N chips for each data symbol is used, then the chip rate of the SS code is N times the data rate. As a result, the spectral bandwidth is increased (spread) by N times the original data spectral bandwidth.
In practice, a (mobile) radio channel is subjected to multipath fading. In this channel, a narrow-band signal experiences flat fading, where all frequency components of the signal drop by the same amount at the same time. As a result, signal level may drop below the threshold value for adequate communication. As the signal bandwidth becomes comparable to or wider than the coherence bandwidth of the multipath channel, the signal experiences frequency-selective fading and the signal level seldom drops below the threshold value, which is an advantage for the wide band signal.
An SS system offers a simple technique that is effective in mitigating the multipath fading. This technique is called a RAKE receiver and uses a filter matched to the channel transfer characteristics. The matched filter in the RAKE receiver outputs, at the sampling instant, a signal obtained by coherently combining the multipath signal components. Since the multipath signal components are subjected to independent fading, the combined signal has a diversity gain. Owing to the high time resolution, an SS system yields the channel impulse response necessary for the matched filter (RAKE) receiver. Thus, the SS RAKE receiver achieves the benefit of wide-band transmission with low effort.
In a mobile communication system, an uplink closed loop power control is used for adjusting a mobile station transmit power in order to keep the received uplink Signal-to-Interference Ratio (SIR) at a given SIR target. The base station is arranged to estimate the total uplink received interference in the current frequency band. The base station then generates TPC (Transmit Power Control) commands according to the relationship between the estimated SIR and the target SIR. If the estimated SIR is larger than the target SIR, then a TPC command “down” is generated. If the estimated SIR is smaller than the target SIR, then a TPC command “up” is generated. Upon reception of a TPC command, the mobile station adjusts its transmit power in the given direction with a step of ΔTPC dB. The step size ΔTPC is a parameter that may differ between different cells.
In WCDMA systems, interference estimation methods can be divided into two classes according to the location of an interference estimator in the RAKE receiver.
If the interference estimator is located before despreading of the received SS signal, it is based on a wide band power measurement. In this kind of solution, the signal plus interference power is measured from the wide band signal, and an interference estimate is then performed by subtracting the signal power from the measured wide band power. However, the subtraction of the signal power is a problem in the WCDMA system, because the data rate might not be known, such that it is difficult to estimate the signal power. Furthermore, this kind of interference estimation does not properly takes an orthogonality of the spreading codes into consideration.
Alternatively, the interference estimator can be located after despreading the received SS signal. In this case, the interference estimation is based on a variance estimation performed at the symbol level. The variance is measured over known symbols assuming that the WCDMA channel is nearly constant during a measuring period. In this kind of estimation, a change of the orthogonality is tracked in the right way. However, due to a small number of known symbols, the variance of that kind of estimator is high. Moreover, fast fading is a problem of this kind of estimation, since changes of the WCDMA channel during the estimation period may degrade the interference estimation.